Active oxygen species for soot oxidation over Ag/ZrO2 were characterized by means of X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption and reduction methods. The temperature-programmed surface reaction (TPSR) between carbon black (CB), a model soot, and surface oxygen revealed that two kinds of adsorbed oxygen species were involved in CB oxidation over Ag/ZrO2. The results of XPS, temperature-programmed desorption of O-2, and temperature-programmed reduction by H-2 were insufficient to distinguish the two kinds of active oxygen species. Temperature-programmed reduction by NH3 (NH3-TPR) resulted in N-2 and N2O formation as products of reduction of the Ag/ZrO2 surface and bulk oxygen. The N2O formation profile in NH3-TPR exhibited two peaks, corresponding to two kinds of oxygen species having a strong oxidation capacity. The effect of Ag loading on the total amount of N2O formation was in good agreement with that on the amount of active oxygen species observed by TPSR-CB. We concluded that NH3-TPR was an appropriate technique for quantitative characterization of the amount of active oxygen species for soot oxidation on Ag/ZrO2. (C) 2012 Elsevier B.V. All rights reserved.